Just a few things that would allow the HFS to be more of a consistent challenge, as well as a bigger player in worldgen.
After the initial wave of clowns and after you breach and conquer the Underworld, there is a small chance every month that the demons may attempt again at breaching your defenses and leaving the Underworld. There are two types:
Demonic Assaults
Demonic Assaults are not all that different from the initial rush that occurs when you first breach the Underworld. Much like a siege, they appear in large numbers at the edge of the map(30-60 demons). However, they can and probably will contain flightless demons, whereas the initial rush only contains flying demons. These flightless demons may even ride non-intelligent flying demons as mounts to fly up to the hole you made.
Demonic Sieges
Demonic Sieges are much rarer variants of Demonic Assaults, as well as being much more !!Fun!!. Unlike Assaults, which are basically a wild horse of demons making a mad rush for it, Sieges are coordinated by unique demons(twisted into humanoid form) and are much more organised. They can only have between 10-30 demons, but most of said demons are either flesh or rocky materials, making them much harder opponents than the weak salt/steam demons ubiquitous in the Underworld. In fact, the coordinator of the siege may even be wearing 1-2 Slade armour pieces, making them EXTREMELY tough opponents.
However, that’s only local. What about world generation? Well...
Demonic Insurrections
During a Demonic Insurrection, hundreds, maybe even thousands of demons attempt a coordinated assault on an Underworld Spire, attempting to force out it’s normal inhabitants and establish it as a secure gateway to the above world, while the goblins and their demonic master attempt to put them down. Demonic Insurrections are VERY rare worldgen events that only occur during worldgen and may only happen per few hundred worlds. If the Insurrection fails, nothing really changes except the goblins lose possibly thousands and are weakened greatly. However, if the Insurrection succeeds, the Underworld Spire(and the Dark Fortress around it) will be permanently routed, causing the change in ages to....
The Age of Despair
“The Age of Despair was a time where the horrors of the Underworld walked the earth.”
(If have not discovered Underworld)
“The Age of Despair was a time where the horrors of the deep walked the earth.”
If a Demonic Insurrection is successful, the world immediately passes into the Age of Despair. All non-intelligent demons will wander into the wilderness to become semi-megabeast like encounters in fortress mode and adventurer mode. However, unique demons will make their own Underworld Spires, which are almost identical to the Underworld Spires made by demon masters, except without a vault and no link to the Underworld. Underworld Spires will have similar naming conventions to Goblin Dark Fortresses, will have a site symbol identical to Dark Fortresses except blood red, and be labeled “The Demonic Underworld Spire of (Name)”. These Spires are hostile to everything around them, even other Spires, though they always prefer to attack living civilisations rather than other Spires. From these Underworld Spires, the demons may gather a religious following, enslave other demons, leave and pose as a diety or even learn the secrets of life and death. All in all, the Age of Despair will be a very !!Fun!! Age for anyone embarking during it.
However, we all know that most demons are stupidly easy to kill. Most of the non-intelligent salt, steam or fire demons will likely slowly be whittled down by civs killing them in encounters, megabeasts and semi-megabeasts purging them, being slaughtered by wildlife or even being killed by their own unique demon brethren. When a sufficient amount of demons(default 80%) are killed, the world passes into the...
Age of Hope
"The Age of Hope was a time when the horrors of the Underworld no longer walked the earth in great numbers."
(If not discovered Underworld)
"The Age of Hope was a time when the horrors of the deep no longer walked the earth in great numbers."